LNG enters the vaporizer from the cylinder body through the pipeline for heating and vaporization. After the pressure stabilizer tank is stabilized, it is stabilized by the gas pressure and then filtered by the gas filter. Then, it can be controlled by the electromagnetic shut-off valve to enter the stabilizer to stabilize. The gas enters the heat exchanger.
CNG enters the pressure reducer from the compressed gas cylinder through the pipeline to reduce pressure to 8bar, and then enters the heat exchanger through the filter. After the gas is heated by the heat exchanger, it enters the FMV through the thermostat, and is controlled by the FMV to enter the mixer and mix with the pressurized air. The electronic throttle controls the mixed gas to enter the engine cylinder for combustion and work.
The LPG comes out of the gas cylinder through the high-pressure solenoid valve to the evaporation pressure regulator, and becomes gaseous LPG. The LPG is fully mixed with air in the mixer through the FTV and enters the engine cylinder for mixed combustion.
◆ ECM electronic control module
ECM is a miniaturized computer management center. It takes signal (data) acquisition as input, performs calculation and processing, analyzes and judges, determines countermeasures, and then issues control instructions, directs actuators to work as output, and provides voltage stabilization to sensors. Power supply or reference voltage. All its functions are completed by various hardware and software. WOODWARD2.0 system uses ECU128-HD microprocessor. Can support single-point or multi-point injection, support CAN communication.
ECM has the following structure: ① Maximum 34 analog inputs, 5 digital inputs, 5 PWM inputs, etc.; ② Maximum support for 12 nozzle drivers, one driver corresponds to one nozzle alone; ③ 11 low-end outputs; ④ 2 CAN Communication port; ⑤ 1 RS-485 communication port. The ECU has two 5V power outputs to supply power to the sensor. The two power sources are independent of each other. If the 5V power supply is short-circuited, the voltage will drop and cause many system errors; one is specially used to connect the sensor and the ECU grounding to ensure accurate sensor readings. The engine electronic control module (ECM) and the ignition control module (ICM) are generally installed in the control box, which is fixed on the frame by the OEM. The engine controller has requirements for waterproof, shockproof, and high temperature resistance. When designing the entire vehicle, the OEM must consider the waterproof, shockproof, and high temperature requirements of the engine controller.
◆ Engine wiring harness
The wiring harness is the nerve of the engine and plays an important role in transmitting signals. The quality of the wiring harness is directly related to the reliability of the engine. If the ECU wiring harness connects the ECU and the engine wiring harness, and has a diagnostic interface, CAN interface and other functional interfaces. The engine wiring harness connects the various sensors and the ECU wiring harness, transmits the signals measured by the sensors to the ECU, and transmits the commands issued by the ECU to the various actuators. The ignition wiring harness transmits the ignition signal of the ICM to the ignition coil to control the point timing. The LNG engine fuel control system is composed of solenoid valves, regulators, gas filters, heat exchangers, thermostats, fuel metering valves, mixers and other components.
The difference between CNG engine and LNG engine lies in the gas conversion device. The high pressure stored in the CNG engine cylinder is 200bar and needs to be decompressed to about 8bar; the pressure value of the liquid natural gas in the LNG engine cylinder after gasification is about 10bar, use a regulator to adjust the pressure to 8bar That's it.
◆ The role of fuel control system
Pressure management: convert the high pressure of the gas cylinder to extremely low pressure before the mixer;
Temperature control: Very low temperature gas will freeze pipelines and components, and the fuel control system will effectively heat and control the gas temperature within a reasonable range.
Injection volume control: The fuel metering valve is equipped with pressure and temperature sensors to provide the ECU with fuel gas temperature and pressure information required for lean burn combustion, and precisely control the injection volume of the nozzle. At the same time, high-pressure gas requires a solenoid valve to control the opening and closing of the gas.
◆ Low pressure solenoid valve (for LNG engine)
The low-pressure solenoid valve is driven by a coil to drive the spool, which is controlled by ECM to open and close, and it is normally closed when it is stopped. The fuel supply can be cut off or restored in time.
◆ Voltage stabilizer
Due to the limitation of LNG characteristics, the opening pressure of the main safety valve is 1.75Mpa, 253psi first, and the opening pressure of the auxiliary safety valve is 2.41Mpa, 350psi, and the cylinder pressure generally does not exceed 1.75Mpa. According to the requirements of the pressure reducing valve, the theoretical workable gas pressure is 0.5-1.72Mpa. Exceeding this pressure range may cause the pressure reducing valve to fail, and the engine cannot be started. Therefore, it is required to install a voltage stabilizer between the filter and the heat exchanger. . Considering the requirement to ensure the normal use of the pressure reducing valve for a long time, the outlet pressure of the general regulator is adjusted to ensure that the pressure of natural gas entering the engine is stable below the set value.
◆ Heat exchanger
The reduction of natural gas from 200bar to 8bar leads to a significant decrease in the gas temperature, and the outlet gas temperature may even drop below -70°C, which will reduce the service life of the pressure reducing valve. The heat exchanger uses the cold liquid of the engine to heat the natural gas. The heat exchanger adopts a cross-flow structure to avoid thermal shock (excessive thermal stress) caused by overcooling of the gas and overheating of the coolant. In all engine working conditions where the cooling water temperature is higher than 0 degrees, the heat exchanger can ensure that the gas is always higher than -40°C, which can prevent gas crystallization before entering the gas metering valve; the cooling water temperature is higher than 82°C, and the gas temperature is higher than 0 To prevent the moisture in the gas from freezing and affecting the performance of the fuel metering valve.
The gas temperature is used to control the coolant flowing through the heat exchanger to control the gas temperature from being too high. Keep the outlet gas at about 0-40°C. When the gas outlet temperature is greater than 60°C, the gas flow will decrease. If the gas temperature exceeds 40°C, the thermostat will turn off within 30 seconds; if the gas temperature is below 10°C, the thermostat will turn on within 30 seconds. The thermostat works.
◆ Fuel metering valve (FMV)
The decompressed medium-pressure natural gas flows through the heat exchanger and the thermostat, is heated to a suitable temperature range, and then enters the fuel metering valve. FMV is equipped with 8∕10∕12 nozzles, divided into 2 groups arranged in parallel, each nozzle has a driver. In normal mode, the nozzles work in turn. Under certain variable conditions, the nozzles work at the same time to speed up the system's response speed. The FMV integrates a natural gas pressure sensor (NGP) and a natural gas temperature sensor (NGT). The decompressed gas flows sequentially through the NGP sensor and the NGT sensor, and then flows through the nozzle for flow control, and finally flows out from the outlet. FMV coils, NGO sensors, NGT sensors and nozzles can all be maintained at the component level. According to the engine operating conditions, the electronic control unit adjusts the fuel metering valve nozzle pulse width duty cycle, controls the amount of gas injection, and ensures that the engine runs at the set air-fuel ratio.